Power magnetic devices, such as inductors and transformers, are employed in many different types of electrical circuits, such as power supply circuits. In practice, most power magnetic devices are fabricated of one or more windings, formed by an electrical member, such as a wire of circular or rectangular cross section, or a planar conductor wound about or mounted to a bobbin composed of dielectric material, such as plastic. In some instances, the electrical member is soldered to terminations on the bobbin. Alternatively, the electrical member may be threaded through the bobbin for connection directly to a metallized area on a circuit board. A magnetic core is typically affixed about the bobbin to impart a greater reactance to the power magnetic device.
As with other types of electronic components, there is a trend in the design of power magnetic devices toward achieving increased power and volumetric density and lower device profile. To achieve higher power, the resistance of the power magnetic device must be reduced, typically by increasing the cross-sectional area of the electrical member forming the device windings, or by simply reducing the electrical path length of the device. To increase the density of the power magnetic device, the bobbin is usually made relatively thin in the region constituting the core of the device to optimize the electrical member resistance. Conversely, the remainder of the bobbin is usually made relatively thick to facilitate attachment of the electrical member to the bobbin terminals or to facilitate attachment of terminals on the bobbin to a circuit board. As a result of the need to make such a bobbin thin in some regions and thick in others, the bobbin is often subject to stresses at transition points between such thick and thin regions.
Another problem associated with present-day power magnetic devices is the lack of co-planarity of the device terminations. Because of the need to optimize the winding thickness of the power magnetic device to provide the requisite number of turns while minimizing the winding resistance, the thickness of the electrical member forming each separate winding of the device is often varied. Variation in the winding thickness often results in a lack of co-planarity of the device terminations, an especially critical deficiency when the device is to be mounted onto a surface of a substrate, such as a printed circuit board ("PCB") or printed wiring board ("PWB").
A surface-mounted power magnetic device is disclosed in U.S. Pat. No. 5,345,670, issued on Sep. 13, 1994, to Pitzele, et al., entitled "Method of Making a Surface Mount Power Magnetic Device," commonly assigned with the present invention and incorporated herein by reference. The power magnetic device of Pitzele, et al. is suitable for attachment to a substrate (such as a PWB) and includes at least one sheet winding having a pair of spaced-apart terminations, each receiving an upwardly rising portion of a lead. The sheet winding terminations and upwardly-rising lead portions, together with at least a portion of the sheet windings, are surrounded by a molding material and encapsulated with a potting material. A magnetic core surrounds at least a portion of the sheet windings to impart a desired magnetic property to the device. Thus, Pitzele, et al. disclose a bobbin-free, encapsulated, surface-mountable power magnetic device that overcomes the deficiencies inherent in, and therefore represents a substantial advance over, the previously-described power magnetic devices. However, several additional opportunities to increase power and volumetric density and lower profile in such power magnetic devices remain.
First, device leads typically extend substantially from the device footprint and therefore increase the area of the substrate required to mount the device. In fact, extended leads can add 30% to the footprint or 50% to the volume of the magnetic device. Second, termination co-planarity requires either the aforementioned devices be molded in a lead frame (requiring additional tooling and tighter tolerances) or the leads be staked in after molding (requiring an additional manufacturing operation). Third, the outer molding compound employed for electrical isolation and thermal conductivity adds both volume and cost and raises device profile.
Accordingly, what is needed in the art is a power magnetic device having an improved termination or lead structure and a structure that attains an acceptable electrical isolation and thermal conductivity without requiring a molding compound. Further, what is needed in the art is a method of manufacture for such devices.